Spread spectrum techniques have proven useful in a variety of communications applications, including cellular telephones, wireless local area networks, and military communications. One advantage of spread spectrum techniques is the ability to build a transmitter which is difficult for an unauthorized user to detect.
Wireless spread spectrum systems use a relatively large amount of spectrum bandwidth to communicate their signals. The large bandwidth is consumed by spread spectrum encoding the message data using a spreading code. The two most common types of spread spectrum encoding are frequency hopping, where a pseudonoise spreading code is used to pseudo randomly change the transmission frequency on a periodic basis, and direct sequence, where the pseudonoise spreading code is used to modulate the transmit signal at a high rate relative to the underlying message data.
In order to detect a spread spectrum transmission, it is generally necessary to know the spreading code beforehand. Furthermore, to extract the message data, it is generally necessary to know the timing of the spreading code. For example, in a direct sequence system, this can be accomplished by knowing the code frequency (rate at which the spreading code advances through its sequence) and the starting time of the spreading code sequence (sometimes referred to as the phase of the code). A signal for which the spread spectrum receiver knows the spreading code, spreading code phase, and spreading code frequency can be referred to as a synchronized signal.
One interesting property of spread spectrum systems is that unsynchronized signals appear as noise to a spread spectrum receiver, and are suppressed by the receiver. This property is sometimes used to provide a so-called spread spectrum multiple access system (also known as code division multiple access). For example, different users can be assigned different spreading codes, in which case a receiver will reject signals from users other than the specific user to whose code the receiver is synchronized. As another example, all users can be assigned a common spreading code, but each user transmits with a different spreading code start time. This results in each user having a different spreading code phase. A receiver tuned to the common spreading code at a particular timing (phase) will reject other users with different code timing (phase). This latter example is sometimes referred to as spread aloha. Spread aloha is particularly advantageous when relatively short messages are to be transmitted from a large number of transmitter units.
Spread aloha can allow a simpler receiver than other forms of code division multiple access, since one common spreading code is used. A spread aloha receiver can search for multiple transmissions using a single correlator to correlate received signals against the common spreading code. Achieving synchronization to the spread aloha signal can, however, still be challenging. For example, generally higher signal to noise ratio is obtained the longer the correlation is performed. Higher signal to noise ratio is desirable because it usually provides a higher detection probability for a given false detection (false alarm) rate. To permit long correlation at the receiver typically requires the transmission of a long preamble. These long preambles reduce the communications efficiency of a spread aloha system. Long preambles also increase the possibility that transmissions from two different transmitters will overlap in time, making reception of both transmissions more difficult.
Additional challenges also exist in synchronizing to a spread aloha. For example, synchronization is typically achieved by comparing the output of the correlator to a detection threshold to determine when a signal is present. Setting this threshold properly can be a challenge. If the threshold is set too high, the probability of detecting a transmission is reduced, as weakly received transmissions may fail to provide a high enough correlator output to exceed the threshold. On the other hand, if the threshold is set too low, there may be false detections where noise causes the correlator output to exceed the threshold even though no transmission is present. If inadequate signal to noise ratio is present at the output of the correlator under desired operating conditions, it can prove difficult or even impossible to set a threshold that provides a desired detection probability and acceptable false detection rate. False detections can be particularly troublesome when they result in the receiver committing to demodulating a message, thus blinding it to additional incoming transmissions.